5-Methylcyclopentadiene

Cyclopentadiene, b.p. 40°, is obtained by heating commercial 85% dicyclopentadiene (e.g., from Matheson, Coleman and Bell Company, Norwood, Ohio) under a short column (M in. diameter X 8-12 in. length) filled with glass helices. The distilled cyclopentadiene is collected in a receiver which is maintained at Dry Ice temperature until the cyclopentadiene is used. Methylcyclopentadiene and other substituted cyclopentadienes such as indene may also be employed for the synthesis of the correspondingly substituted ferrocenes. In these cases, the reaction of the hydrocarbon with sodium is much slower than with cyclopentadiene, and refluxing for several hours is required to complete the reaction. 

Similarly, examine the overlap between the HOMO of 1-methylcyclopentadiene and the LUMO of an acrylonitrile BF3 complex. Does the Lewis acid affect overlap Would you expect BF3 to enhance, retard, or leave unchanged the rate of Diels-Alder addition ... 

Check your predictions by calculating activation energies for Diels-Alder additions. Data for transition states 1-methylcyclopentadiene+acrylonitrile and 1-methyl-cyclopentadiene+acrylonitrile BF3 are available. 

HOMO for 1 -methylcyclopentadiene donates electrons in a Diels-Alder addition. 

Earlier in this section we made reference to the azo coupling of triphenylphos-phonium cyclopentadienylide (12.109). Makhailov et al. (1984) found that penta-methylcyclopentadiene (12.121) reacts with mono-, di-, and trinitrobenzenediazo-nium salts to give a mixture of 12.122 and 12.123, formed by arylazo substitutions at the sp3-hybridized carbon of the five-membered ring and at the methyl group attached to the same carbon atom, respectively. This is definitely not a classical azo coupling ... 

Coxon et al. also reported that AMI calculation of the transition states for the reactions between ethylene and 5-methylcyclopentadienes 45-49 bearing the... 

Methylcyclopentadienyl manganese tricarbonyl that has been suggested as a fuel additive is decomposed primarily by photolysis in aqueous medium. This resulted in the formation of methylcyclopentadiene that may plausibly be presumed to polymerize, and a manganese carbonyl that decomposed to Mn304 (Garrison et al. 1995). 

The THF-solvated product of the ligand exchange reaction between bis(penta-fluorophenyl)ytterbium and methylcyclopentadiene was explosive. The expected product, bis(methylcyclopentadiene)ytterbium, obtained by another exchange route, is not explosive. 

Apparently 9-phenylfluorenyl is not only a good anion but also a bad cation. The chloride probably shares in some of the resonance stabilization of the anion while the cation does not. Another example of a connection between the resonance of an anion and the properties of a related covalent compound is provided by the hydrocarbon triphenyl-methylcyclopentadiene, which has an unusually high dipole moment although it does not conduct in liquid sulfur dioxide.180... 

Methylcyclopentadiene (mecp) clearly illustrates the roles of Y and L. CpCo(Tj -mecpH) has the same activity at 150°C as other cpCoL complexes, however, (7 -mecp)Co(cod) has lower activity than (cp)Co(cod) for a given temperature (Fig. 3) and shows different selectivity (85TH2). Hence, in order to optimize the activity and selectivity of the catalyst, an appropriate choice of the ligand Y must be made. 

Regio and stereochemical preferences in kinetically-controlled reactions may also be expressed as isodesmic processes. For example, the regioselectivity of (endo) addition of 2-methylcyclopentadiene with acrylonitrile comes down to the difference in energy the transition states leading to meta and para products, respectively. 

Similarly, the difference in energy between syn and anti transition states for endo) addition of 5-methylcyclopentadiene with acrylonitrile accounts for the stereochemistry of this reaction. 

All levels of calculation (including semi-empirical calculations) provide a qualitatively correct account of the experimental regio and stereochemical preferences. The only (apparent) exceptions are that both B3LYP/6-31G and MP2/6-31G models show modest preferences for meta products in cycloaddition of 2-methylcyclopentadiene and acrylonitrile. Note, in particular, the success of the calculations in properly assigning the more crowded syn product for the cycloaddition of 5-methoxycyclopentadiene and acrylonitrile. Also note the large magnitude for the preference. Clearly factors other than sterics are at work. 

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